After performing the usual* test when getting a System.FormatException it seems that the I18n has not yet been done properly in Windows Live Writer. Basically (it seems like) the code assumes . (dot) as a decimal separator somewhere, but gets a , (comma) since the Swedish (Finland) locale (sv-FI) uses comma as decimal separator.
Workaround: Change the decimal separator to a dot.
Usual test: Change the regional settings to use English (United States)